In a typical cellular network, also referred to as a wireless communication system or simply a system, a User Equipment (UE), communicates via a Radio Access Network (RAN) to one or more Core Networks (CNs).
A user equipment is a mobile terminal by which a subscriber may access services offered by an operator's core network and services outside the operator's network to which the operator's RAN and CN provide access. The user equipment may be for example communication devices such as mobile telephones, cellular telephones, smart phones, tablet computers or laptops with wireless capability. The user equipment may be portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another user equipment or a server.
The user equipment is enabled to communicate wirelessly in the system. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between the user equipment and a server via the radio access network and possibly one or more core networks, comprised within the system.
The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g. a Radio Base Station (RBS). The base station is, in some radio access networks, also called evolved NodeB (eNB), NodeB, or B node. A cell is a geographical area where radio coverage is provided by the base station at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment within range of the base station.
Standardised by the third Generation Partnership Project (3GPP), High Speed Packet Access (HSPA) supports the provision of voice services in combination with mobile broadband data services. HSPA comprises High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA) and HSPA+. HSDPA allows systems based on the Universal Mobile Telecommunications System (UMTS) to have higher data transfer speeds and capacity. In HSDPA, a new transport layer channel, High Speed Downlink Shared CHannel (HS-DSCH), has been added to the 3GPP UMTS Release 5 and further specifications. It is implemented by introducing three new physical layer channels: High Speed-Shared Control CHannel (HS-SCCH), Uplink High Speed-Dedicated Physical Control CHannel (HS-DPCCH) and High Speed-Physical Downlink Shared CHannel (HS-PDSCH). The HS-SCCH informs the user equipment that data will be sent on the HS-DSCH, two slots ahead. The HS-DPCCH carries acknowledgment information and a current Channel Quality Indicator (CQI) of the user equipment. This value is then used by the base station to calculate how much data to send to the user equipment in the next transmission. The HS-PDSCH is the channel mapped to the above HS-DSCH transport channel that carries actual user data. HSPA may recover fast from errors by using Hybrid Automatic Repeat reQuest (HARQ). HARQ is a technique that enables faster recovery from errors in systems by storing corrupted packets in the receiving device rather than discarding them. Even if retransmitted packets have errors, a good packet may be derived from the combination of bad ones. The term downlink used above may be abbreviated DL and is the link seen in the direction from the base station to the user equipment.
Multiple Input Multiple Output (MIMO) refers to any communications system with multiple antennas at the transmitter and receiver, and it is used to improve communication performance. The terms input and output refer to the radio channel carrying the signal, not to the devices having antennas. At the transmitter (Tx), multiple antennas may be used to mitigate the effects of fading via transmit diversity and to increase throughput via spatial division multiple access. At the receiver (Rx), multiple antennas may be used for receiver combining which provides diversity and combining gains. If multiple antennas are available at both the transmitter and receiver, then different data streams may be transmitted from each antenna with each data stream carrying different information but using the same frequency resources. For example, using two transmit antennas, one may transmit two separate data streams. At the receiver, multiple antennas are required to demodulate the data streams based on their spatial characteristics. In general, the minimum number of receiver antennas required is equal to the number of separate data streams. 4×4 MIMO, also referred to as four branch MIMO, or a system in MIMO mode, may support up to four data streams.
Several new features are added for the long term HSPA evolution in order to meet the requirements set by the International Mobile Telecommunications-Advanced (IMT-A). The main objective of these new features is to increase the average spectral efficiency. One possible technique for improving downlink spectral efficiency would be to introduce support for four branch MIMO, i.e. utilize up to four transmit and receive antennas to enhance the spatial multiplexing gains and to offer improved beam forming capabilities. Four branch MIMO provides up to 84 Mbps per 5 MHz carrier for high Signal to Noise Ratio (SNR) user equipment and improves the coverage for a low SNR user equipment.
Channel feedback information enables a scheduler located for example in a base station, to decide which user equipment should be served in parallel. The user equipment is configured to send three types of channel feedback information to the base station: CQI, RI and PMI. CQI is an important part of channel information feedback. The CQI provides the base station with information about link adaptation parameters which the user equipment supports at the time. The CQI is utilized to determine the coding rate and modulation alphabet, as well as the number of spatially multiplexed data streams. RI is short for Rank Indicator or Rank Information and is the user equipment recommendation for the number of layers, i.e. streams to be used in spatial multiplexing. The skilled person will understand that this is equivalent to the number of transport blocks preferred. RI is only reported when the user equipment operates in MIMO mode with spatial multiplexing. The RI may have the values 1 or 2 in a 2×2 MIMO configuration and it may have the values from 1 and up to 4 in a 4×4 MIMO configuration. The RI is associated with a CQI report. This means that the CQI is calculated assuming a particular RI value. The RI typically varies more slowly than the CQI. PMI is short for Precoding Matrix Indicator and provides information about a preferred precoding matrix in a codebook based precoding. PMI is only reported when the user equipment operates in MIMO. The number of precoding matrices in the codebook is dependent on the number of antenna ports on the base station. For example, four antenna ports enables up to 64 matrices dependent on the RI and the user equipment capability. A Precoding Control Indicator (PCI) indicates a specific precoding vector that is applied to the transmit signal at the base station.
FIG. 1 shows the messages exchanged between a base station and a user equipment during a typical data call set up. The method comprises the following steps, which steps may be performed in any suitable order:
Step 101
The base station broadcasts the Common Pilot Indicator CHannel (CPICH) which is a downlink channel with constant power and of a known bit sequence so that the user equipment may estimate the channel and compute the CQI and the PCI in step 102.
Step 102
From the CPICH in step 101, the user equipment estimates the channel and computes the CQI and the PCI.
For two antennas, the CQI is computed as follows:
  CQI  =      {                                                      15              ×                              CQI                1                                      +                          CQI              2                        +            31                                                                                                when                  ⁢                                                                          ⁢                  2                  ⁢                                                                          ⁢                  transport                  ⁢                                                                          ⁢                  blocks                  ⁢                                                                          ⁢                  are                  ⁢                                                                          ⁢                  preferred                                                                                                      by                  ⁢                                                                          ⁢                  the                  ⁢                                                                          ⁢                  UE                                                                                                      CQI            S                                                                                                when                  ⁢                                                                          ⁢                  1                  ⁢                                                                          ⁢                  transport                  ⁢                                                                          ⁢                  block                  ⁢                                                                          ⁢                  is                  ⁢                                                                          ⁢                  preferred                                                                                                      by                  ⁢                                                                          ⁢                  the                  ⁢                                                                          ⁢                  UE                                                                        
Where the CQI is the channel quality per individual layer. CQI1 represents the CQI of the first codeword, CQI2 represents the CQI of the second codeword and CQIS represents the CQI of the single stream. The number 31 is used to differentiate between two codewords and one codeword. If the CQI is less than 31, it is one codeword transmission.
Step 103
The information computed in step 102, i.e. the CQI and PCI, along with a HARQ ACK/NAK is reported to the base station using the HS-DPCCH. The periodicity of HS-DPPCH is one subframe, e.g. 2 msec. The HS-DPCCH in the 3GPP Release 5 to Release 9 is based on a 1×SF256 solution. SF is short for Spreading Factor. The structure of the HS-DPCCH is shown in FIGS. 2a and 2b. FIG. 2a illustrates a general location of the PCI and CQI in the structure and FIG. 2b illustrates and example of how the PCI and the CQI are located in the structure. As well-known, the HS-DPCCH subframe structure comprises one slot for HARQ ACK/NACK transmissions and two slots for CQI/PCI transmissions. In the following, even though the text or the drawings refer to a HARQ ACK, it is appreciated that this may also be a HARQ NACK. ACK is short for ACKnowledgement and NACK is short for Not ACKnowledgement.
The HS-DPCCH subframe structure in FIGS. 2a and 2b for a Transmission Time Interval (TTI)=2 ms comprises a HARQ ACK or NACK which notifies the base station that the user equipment has received correct downlink data or not. The field defines like this: 1-NACK, 0-ACK, i.e. 1 represents NACK and 0 represents ACK. The CQI reflects the PCI based on CPICH strength. Each subframe comprises a HARQ ACK/NACK, two CQI-fields and one PCI field. In other words, every subframe comprises the same fields.
For the 3GPP Release 7 which covers MIMO, the HARQ ACK/NACK codebook comprises six codewords plus PREamble/POSTamble (PRE/POST) information.
In the 3GPP Release 7 there are 5 or 2×4 bits allocated for describing the CQI depending on the CQI type. There are 30 or 15 CQI values per stream for rank 1 and rank 2, respectively. The rank may vary from one up to the minimum of number of transmit and receive antennas. The rank determines how many layers, also referred to as the transmission rank, which may be successfully transmitted simultaneously. The rank is implicitly signalled via the CQI. Furthermore CQIs for each stream are signalled independent of each other. In addition to CQI bits there are 2 bits allocated for signaling the preferred precoding information. The seven (or ten) information bits are then encoded into twenty channel bits that are transmitted during the second and third slot.
Returning to FIG. 1.
Step 104
Once the base station receives the CQI, PCI and HARQ ACK/NACK, it allocates the required channelization codes, modulation and coding, precoding channel index to the user equipment after scheduling to be used for the downlink transmission.
Step 105
The base station transmits the information about the allocated channelization codes, modulation and coding, precoding channel index from step 104 to the user equipment using the HS-SCCH.
Step 106
The user equipment detects the transmission on the HS-SCCH, i.e. the user equipment's receives the information transmitted in step 105.
Step 107
Once the user equipment has detected the transmission on the HS-SCCH, the base station starts its downlink transmission to the user equipment through a data traffic channel using the HS-PDSCH. The base station periodically transmits to the user equipment for every TTI, which is 2 msec in HSDPA.
In general, HS-DPCCH design depends on many factors like the number of codewords supported, the number of HARQ processes, the precoding codebook etc. Four branch MIMO should support two codewords and two HARQ processes.
The current HSDPA system (3GPP Release 7-10) supports one or two transmit antennas at the base station. For these current systems, the user equipment measures the channel from channel sounding and reports the channel state information in one sub frame. A sub frame may be defined as for example one TTI which may be e.g. 1 ms or 2 ms. Typically this report consists of the CQI which explicitly indicates the RI and the PCI. The user equipment sends this report periodically for every subframe, i.e. for every TTI to the base station. Once the base station receives this report it grants the Modulation and Coding Scheme (MCS), number of codes, rank and the PCI to each specific user equipment based on a scheduler metric. Based on this information, the base station may optimize the downlink throughput for each TTI.